This invention relates to delignifying pulp in the presence of oxygen, and more particularly to a process for oxidative delignification of a medium consistency pulp using a series of tubular reaction zones.
Conventional processes for chemical pulping of fibrous raw materials have in the past utilized sulfur-containing compounds while conventional bleaching processes have utilized chlorine containing compounds. Today, environmental considerations have resulted in a search for nonpolluting processes which can offer the desired pulp yields and qualities. Much attention has been devoted to the use of oxygen in combination with alkaline chemicals to delignify pulp.
For example, several workers have investigated oxygen delignification of high consistency pulp (i.e., 20-30% consistency). See, Eachus, TAPPI Volume 58, p. 151-154 (September 1975) and Hasvold, 1978 International Sulfite Conference, Montreal, Canada (Sept. 13, 1978). Other workers have utilized oxygen delignification in low consistency (i.e., 1-5% consistency) pulping or bleaching processes. See, Paper Trade Journal p. 37-39 (July 15, 1978).
However, both of these processes suffer from several disadvantages. Low consistency operation requires a large reactor volume to maintain an acceptable retention time for the pulp. Operating at low consistency also produces large power demands for pumping large volumes of pulp and a high steam usage to heat the pulp in the reactor. Additionally, the low concentrations of dissolved solids in the spent liquor increases evaporation costs for chemical recovery processes. Operation at high consistency, on the other hand, usually requires special dewatering equipment to attain the higher consistency. It is also known that high consistency operation of an oxygen delignification system can result in overheating of the pulp due to the exothermic delignification reaction, as well as pulp degradation and even combustion of the pulp.
Carrying out oxygen delignification of pulp at medium consistency (i.e. 8-20% consistency) would be advantageous in that much existing mill equipment, including pulp washing and thickening equipment, is designed to operate in that consistency range and no special equipment would be required to attain that range. Markham et al, in copending application Ser. No. 072,796, filed Sept. 5, 1979 utilize a medium consistency system to delignify pulp mill rejects. Some workers have reported satisfactory results operating at medium consistency on a laboratory scale using rotary autoclaves with no internal means of mixing (See, e.g., Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979; Saukkonen et al, TAPPI Volume 58, p. 117 (1975); and Chang et al, TAPPI Volume 56, p. 97 (1973)). However, such equipment is not suitable for scale-up to handle large tonnages of pulp on a commercial scale. Other workers have encountered serious problems even on a small laboratory scale. For example, Eachus, TAPPI Volume 58, p. 151 (1975), reported that oxygen delignification at medium consistency was not practical because of a high alkali requirement, oxygen starvation, and a limited delignification.
Chang et al, TAPPI Vol. 57, p. 123 (1974), concluded that operation at medium consistency produced a considerably lower delignification rate than high consistency operation and also resulted in nonuniform delignification. Although the authors suggested that these problems could be overcome through the use of higher oxygen pressures in the reaction vessel, use of such higher pressures has several disadvantages. These include greater costs for a thicker-walled reaction vessel, greater difficulty in feeding pulp against the higher pressure, and an increased danger of gas leakage. Vertical tube oxygen reactors operating at medium consistency have been constructed for trial purposes. (See Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979, and Kleppe et al, TAPPI Vol. 59, p. 77 (1976).) However, such vertical tube designs have serious deficiencies, including channeling of gas and pulp up through the tower and also the requirement for a high speed mechanical mixer to disperse oxygen into the pulp slurry. Such high speed mixing can lead to pulp degradation and additionally requires substantial power input.
As can be seen, there is a need in the art for a simple and efficient process for oxygen delignification of medium consistency pulp which avoids the problems which have plagued the prior art.